1. Field of the Invention
The present invention relates to structures which are unidirectionally stable with respect to thermal and moisture expansion. In other words, the structures have an overall coefficient of thermal expansion and moisture expansion of approximately zero in one direction. More particularly, but without limitation, the present invention relates to longitudinal scales which are dimensionally stable along the length thereof.
2. Description of the Prior Art
There is a great need for and a wide variety uses for products and structures which are dimensionally stable despite significant changes in their temperature. Many products which are subjected to wide variations in temperature must have coefficients of thermal expansion near zero in order to reduce the stresses and strains on the connected elements of these products. In other products, small movements due to temperature changes are unacceptable to the use of the product. For example, in the fields of measurement or position detection, movements of the elements of the product due to thermal expansion can cause errors in the measurement or position detected. This is especially true of longitudinal scales which are used in position detection.
In the past, the most commonly used dimensionally stable product has been fused silica glass of a type which has a low coefficient of thermal expansion. For example, glass scales etched with position markings have been commonly used in position detectors when extreme accuracy is required. A problem, however, with the fused silica glass is that it is expensive, difficult to machine or form, difficult to mark and brittle in its usage. Moreover, many applications require a product which is not transparent which eliminates the use of this type of glass.
Efforts to make other dimensionally stable products have relied on balancing laminations of different thermal expansion materials. A problem with these products, however, is that the connection between the laminations is difficult to achieve and can result in warping of the structure if many laminations are not used.
Another effort at dimensionally stable structures has been the composite fiber-resin materials. It is known that carbon fibers have a negative coefficient of expansion. Resins which have a positive coefficient of expansion can be bonded to the fibers to form a composite structure which is dimensionally stable. For example, a sheet of randomly oriented carbon fibers can be impregnated with a resin such as polyphenylene sulfide and then laminated with several other similarly constructed sheets oriented in different directions to further randomize the orientation of the fibers. The resulting laminations can be fused by a heated press to form a composite structure of low coefficient of thermal expansion. However, because of the hardness of the carbon fibers, this material is relatively difficult to cut or machine. Still further, because the randomization is not perfect and because of variations in densities in various locations on the sheet, it is not possible to achieve a structure which is as dimensionally stable with respect to thermal expansion as desired.
Although dimensionally stable carbon/epoxy composite materials have been successfully applied in outer space applications, they are subject to moisture expansion effects on earth which make them unsuitable for such application.
It is accordingly an object of the present invention to provide a structure which is unidirectionally stable with respect to thermal expansion, i.e., it has a very low distortion due to thermal and moisture effects along a single dimension of the structure. It is also an object of the present invention to provide a method for fabricating this unidirectionally stable structure in a repeatable manner. Still further, it is an object of the present invention to provide a longitudinal scale which is stable with respect to thermal and moisture expansion along the length thereof and will retain these properties over time.